Cationic starch compositions have been long applied as an additive in paper manufacturing.
For instance, U.S. Pat. No. 6,451,170 discloses a process for obtaining such compositions. The starch can be selected from a variety of starches, including corn, potato, tapioca, wheat, rice, etc. It is preferably a corn starch, and typically a dent corn starch (column 5, line 31). The starch is typically cross-linked with a cross-linker which is reactive with the hydroxyl functionality of the starch. The cross-linker permits a greater range in particle sizes compared to non-cross-linked starch. The starch compositions are desirably cationic starches that retain a positive charge when dissolved in water. The starch preferably contains a quaternary ammonium ion, which gives enhanced flexibility in pH. Frequently, such quaternary ammonium-containing starch is derivatized by etherification of hydroxyl groups with an appropriate etherifying agent, such as (3-chloro-2 hydroxypropyl) trimethyl ammonium chloride, the methyl chloride quaternary salt of N-(2,3-epoxypropyl) dimethylamine or N-(2,3-epoxypropyl) dibutylamine or N-(2,3-epoxypropyl)methylaniline. The starch is typically cationized to a degree of substitution (DS), as known per se, of greater than 0.005, but not greater than 0.100, preferably from 0.030 to 0.040. Cooking is thereafter applied to set the average particle size in the resulting cationic starch composition. It is herein expressed, as a general rule that an increase in cooking temperature results in a reduction of the average particle size. The cooking temperature, and other conditions thereof, such as pressure, may be tuned to arrive at a particle size of the starch that is comparable to the particle size of paper fibers.
Various types of starch have been proposed as an alternative to the usual corn, waxy corn or potato starches. US2006/0254737 proposes the use of blends of cationic starches. EP0139597 describes bulk paper additives made from mixtures of at least one cationic cereal starch and at least one cationic tuber starch. Surprisingly and unexpectedly, these are capable of exhibiting synergies, in particular in terms of physical and retention characteristics. US2006/0225855 specifies that potato starch is currently the most applied starch for use in paper manufacturing. Its use is preferred over corn starch, and wheat starch. However, the supply of potato starch is becoming difficult, mainly because of costs of extraction and the regulations applied to them. Therefore, US2006/0255855 proposes the use of pea starch.
In addition to the choice of the type of starch, the cross-linking degree is highly relevant to its properties as an additive. EP0603727 discusses the impact of the degree of cross-linking on the properties of paper additives. The properties of corn and waxy maize starch, that are per se less than those of cationic potato starches, may be improved however with the combination of cross-linking and jet cooking (Table 5). However, there is also a maximum amount of useful cross-linking: when increasing the concentration of cross-linker from 0 to 0.02% epichlorohydrin by weight of (waxy corn) starch, the peak viscosity decreases and the viscosity breakdown upon cooking reduces to zero. Above a cross-linking degree of more than 0.02%, the cooking increases the viscosity rather than reduces it.
WO02/088188 nevertheless proposes cross-linking with as much as 3% TMF cross-linker by weight of starch. It teaches the addition of water during cross-linking so as to arrive above the equilibrium moisture. It shows that cross-linking, without cationization, may give rise to a substantial viscosity decrease. The background hereof is likely that the additional moisture is needed so as to allow a better diffusion of the TMF cross-linker through the starch. The cross-linking thereafter leads to granulates, such that the starch concentration in the water phase and therewith the viscosity decreases. Not surprisingly, Example 7 refers to filtration of a starch cake. It is however unclear how such a starch could be a good additive in the wet end of paper manufacturing. EP1360209 continues on that path by proposing the extrusion of such a very highly cross-linked starch (0.1-0.8% cross-linker)
Furthermore, the cooking conditions are relevant. EP0623727 observes that jet cooking is preferred over batch cooking, because it leads to higher calcium carbonate retention. It suggests a cooking temperature of 105 to 120° C. With high cross-linking as discussed above, incomplete gelatinization may occur during cooking. When applying that to a paper machine, dewatering turns out problematic. WO97/46591 proposes a variation thereon, which apparently works for specific applications. It proposes jet cooking of cationic (DS=0.016) and cross-linked starches at a low temperature, for instance 70° C. This low temperature is used to control the degree of swelling such that over-swelling or bursting does not occur during this cooking. Complete gelatinization is to occur only after being applied to the paper furnish and subsequently heated over dryers. Suitable thereto are a selection from the group of a blend of equal parts wheat and tapioca starch, tapioca starch or wheat starch.
This process of adding only swollen granules to the paper pulp and subsequently allowing gelatinization however does not allow a high paper production rate. The patent application states that the production rate was not lower than before, but recycled paper mills in the 1990s typically were used to produce thick paper particularly for packaging applications. Such type of paper was made on very slow machines compared to today's advanced paper mills for standard and high-quality paper. WO97/46591 also appears to admit that when stating that the paper machine speed may be improved on heavier grades of paper.
It is therefore still an object of the invention to develop alternatives for potato starches which are suitable for use in advanced paper mills. Starch compositions are used as additives in the papermaking process to improve drainage and retention, to add strength, and to improve formation properties of the paper. Drainage, or de-watering ability, is an important consideration in the manufacture of paper, because it is related to how fast a paper machine can remove water from the web. Typically, improved dewatering corresponds to higher speeds on paper machines and to higher production rates of paper. Furthermore, it is desired to increase the loading of cationic starch into the paper manufacturing without a negative impact on the paper formation, i.e. the resulting paper quality.